When signals have a frequency in the radio frequency (RF) range or higher, ordinary electrical cables are insufficient. Transmission lines, however, are known to carry such signals with reduced reflections and power losses. Transmission lines include coaxial cables, optical fibers, waveguides, striplines and microstrips. Generally, transmission lines are used when the frequency of the signal is high enough that the wavelength of the signal approaches the length of the cable. Multi-layer integrated transmission line circuits, such as striplines and microstrips, are used for various applications, such as connecting high speed communication devices and distributing cable television signals.
A stripline circuit is a transmission line that uses a conducting strip of metal positioned between two parallel ground (reference) and/or power planes that provide an AC current return path. The insulating material of the substrate forms a dielectric. The width of the strip, the thickness of the substrate and the relative permittivity of the substrate determine the characteristic impedance of the stripline. A microstrip circuit is a transmission line that uses a conducting strip of metal separated from a single reference plane by a dielectric layer (substrate).
While existing transmission lines, such as striplines and microstrips, have reduced the insertion loss for signals in the RF range and higher, additional robustness to signal loss is required to satisfy the insertion loss targets that are required for evolving higher data rate systems. For example, as data rates are moving towards 28 GBps, signal losses have become even more important in the total system budget. Tangent loss of the dielectric material in the substrate is a significant contributor to overall loss in the substrate. Tangent loss tends to be lower in integrated circuit (IC) packages fabricated using ceramic materials than in IC packages fabricated using organic materials. Generally, however, ceramic IC packages are significantly more expensive than organic IC packages. A need therefore exists for improved techniques for reducing the signal losses in multi-layer integrated transmission line circuits, such as striplines and microstrips.